The invention relates to a piston pump which is intended for use in a hydraulic vehicle brake system.
Such piston pumps are known per se; see for instance German Patent Disclosure DE 41 07 979 A1. The known piston pump has a piston, which is axially displaceably received in a pump housing. By means of an eccentric element drivable to rotate by an electric motor, the piston can be driven to execute an axially reciprocating stroke motion. To control a fluid flow through it, the known piston pump has two check valves, one of which forms an inlet valve and the other of which forms an outlet valve. The outlet valve closes under spring loading; the inlet valve can close under spring loading but can also be without a closing spring, especially if the piston pump is self-aspirating.
To attain good hydraulic pump efficiency and to reduce noise, it is important that the outlet valve of the piston pump quickly close at the dead center point of the piston at the end of a supply stroke. The supply stroke is the piston motion in a direction that reduces a volume of a positive displacement chamber of the piston pump and as a result positively displaces fuel, aspirated previously during an intake stroke, from the positive displacement chamber. Rapid closure of the outlet valve requires a valve closing spring with high spring force, which in known piston pumps brings about an opening pressure of the outlet valve of over 1 bar, for instance.
Modern vehicle brake systems with slip control are now typically evacuated before being filled with brake fluid. Filling a vehicle brake system without prior evacuation would lead to air inclusions in voids of the vehicle brake system, voids that are formed by magnet valves, hydraulic reservoirs, dampers, and angled fluid bores that form blind bores; the vehicle brake system would be nonoperational. Simple filling of the vehicle brake system with positive displacement of the air contained in it by inflowing brake fluid without prior evacuation is not possible. From the above, it is also clear that all the voids in the vehicle brake system must be evacuated as completely as possible before the vehicle brake system can be allowed to be filled with brake fluid; there must not be any air trapped in voids of the evacuated vehicle brake system. Air remaining in voids can in some cases cause an unpleasantly soft behavior of the brake pedal.
If the known piston pump, on filling of a hydraulic vehicle brake system of which the piston pump is a component, is evacuated from the inlet and/or the outlet side, then a vacuum on the inlet side creates a pressure difference between the positive displacement chamber of the piston pump and the pump inlet, and this difference closes the inlet valve. The positive displacement chamber cannot be evacuated through the inlet valve, regardless of whether the inlet valve is spring-loaded or springless. If the piston pump is evacuated from the outside, a pressure difference of at most 1 bar is operative, which urges the outlet valve in the opening direction. The pressure difference is the result of the pressure, corresponding to the ambient air pressure, of the air trapped in the positive displacement chamber and the vacuum on the outlet side of the piston pump. This pressure difference is not capable of opening the outlet valve counter to the force of the valve closing spring of the outlet valve; that requires an opening pressure of more than one bar. Consequently the air remains trapped in the positive displacement chamber, and the positive displacement chamber of the piston pump is not evacuated. Filling the positive displacement chamber by operating the piston pump with brake fluid may possibly succeed, in which case the trapped air can be positively displaced out of the positive displacement chamber. However, it can also happen that the air trapped in the positive displacement chamber will remain there even in a piston pump operation, in which case the piston pump will not pump.
The problem of air inclusion in the positive displacement chamber of the piston pump is especially severe in evacuation of the vehicle brake system with piston pumps that have a piston restoring spring in the positive displacement chamber. In these piston pumps, an idle space, that is, the residual volume of the positive displacement chamber at the end of the supply stroke, is especially large, since the piston restoring spring requires a residual minimum height of the positive displacement chamber in the axial direction.